Transparent armor against bullets has been in widespread use for many years. Such so-called "bullet-proof" windows vary from thick plate glass layers bonded together, to complex sandwiches of plastic layers bonded to each other or to glass. Such sandwiches must be completely transparent, have no optical distortion, have maximum ballistic resistance and be as light-weight as possible to be fully satisfactory. Depending on the use of such armor, it may have to be resistant to vibration and shock, scratching and adverse environmental conditions such as low and high temperatures, rain, frost and snow.
Many of the functions of the various layers of the sandwich are well defined. The characteristic properties of glass sheet which may form such a layer are its strength as well as its abrasion resistance. This abrasion resistance is essential in particular in outdoor and/or vehicular applications where highly abrasive particulates such as aluminum oxide and silicon oxide in dust are encountered at slow and high speeds. Because of the hardness of such dust, only glass which is of similar or greater hardness than the dust can withstand the abrasive impact of such dust without losing its transparency. The plastic solid layer serves the purpose of absorbing energy via its properties of high impact resistance.
The interlayer between the glass and plastic layer serves two or three purposes depending on whether identical or non-identical materials (e.g. two glass sheets, two polycarbonate sheets or one sheet of each) are bonded.
Between non-identical materials the purpose of the interlayer is to:
(a) merely provide adhesion between the layers, in which case such a layer would characteristically be of thickness less than 1 mm;
(b) provide an energy absorbing path in the transition zone between hard and brittle material such as glass and the flexible and ductile material such as polycarbonate;
(c) provide a stress-free transition zone between the two solid layers which are of widely different linear expansion coefficients.
It is evident that the development of a suitable sandwich is an extension of the prior art of manufacturing safety glass by laminating sheets together, but the stringency of the requirements for bullet proof properties far exceed those for motor vehicle safety glass.
This has led to continual effort to improve such laminates. For instance, U.S. Pat. No. 3,658,636 published in 1972 provided a safety glass composite which would meet both the requirement of motor vehicles and also be able to withstand the impact of bullets. It tackled the difficulties of replacing the glass inner sheet of the laminate with plastic, whilst acknowledging an appreciation of the advantages of doing this. U.S. Pat. No. '636 thus deals with composites obtained by bonding glass to polyamide with a plasticized polyvinyl butyral layer having a preferred thickness of 0.76 mm by pressing together the sandwich under heat. As a test for the bullet resistance this patent describes the extent of damage to a 20.times.20 cm panel by a 9 mm bullet fired under specific conditions.
However, the method of producing the laminate of U.S. Pat. No. '636 has various disadvantages as described in U.S. Pat. No. 4,297,185. For instance the method of U.S. Pat. No. '636 is time consuming and expensive because in addition to the time spent for heating, additional time must be spent for slow cooling the pieces to prevent possible breakage due to thermal shock. Moreover, the adhesives have high viscosities which make the elimination of air bubbles a problem.
A further development is described in U.S. Pat. No. 3,671,370 (published in 1972) which points to the use of polycarbonate as the plastic layer. It also describes the function of polyvinyl butyral previously mentioned as not only an adhesive but as a resinous interlayer or transition material for helping in the absorbing of the energy of the bullet. In addition, it describes a further transition material, a two component polyurethane. The claimed multiple safety glass unit is indicated as having been tested for penetration by subjecting it to a 50 caliber armor piercing projectile. However, the use of a two component polyurethane as described in U.S. Pat. No. '370 has evident disadvantages. The material has to be heated under vacuum before and after the addition of the curing agent to eliminate air, and curing conditions given are for a long period at a high temperature (22 hours at 175.degree. F.) suffering from the same disadvantage as previously mentioned.
Attempts have been made to simplify the production of the glass/plastic laminates by using adhesives cured by U.V. radiation. The above mentioned U.S. Pat. No. 4,297,185 (October 1981) and others (e.g. U.S. Pat. No. 4,355,077) describe U.V. curable compositions comprising polyurethane pre-polymers, co-polymerizable monomers and photo initiators. However, these layers do not appear to have the characteristics of transition layers as they are required to be relatively thin with a preferred thickness defined by a weight of 5-10 g/m.sup.2 going up to a maximum thickness of 200 microns. Moreover, although these materials are described for use in transparent armor, no examples show tests for the impact of bullets, indicating that the function of this particular layer within the system is primarily adhesive and not energy absorbing.
The Lee U.S. Pat. No. 4,841,372 is directed to a cathode ray tube contrast enhancement system and discloses a resin bonding system for bonding a flat implosion protection panel to the flat face plate of a cathode ray tube, the resin bonding system being designed for differential adhesion so that the face plate separates more easily from the resin than does the implosion protection panel. The adhesive in question is provided in two separate layers having different adhesive properties, the outer resin layer adhering tightly to the implosion panel and the inner layer 30 adhering to the face plate and weakly to the outer layer. The outer layer comprises a mixture of a multifunctional urethane acrylate oligomer, a monofunctional acrylic monomer and optionally one or more of a difunctional acrylic monomer and trifunctional acrylic monomer, and critically at least 1% by weight of an organic dye which constitutes a neutral density filtering agent. The inner layer, designed to adhere less strongly comprises a mixture of a multifunctional urethane acrylate oligomer, a monofunctional acrylic monomer, a releasing agent and optionally a difunctional acrylic monomer and a trifunctional acrylic monomer.
The U.S. Pat. No. 4,938,831 in the name of Wolf discloses a bonding method for preparing automotive headlamp assemblies using a light curable adhesive composition consisting of a low molecular weight aliphatic urethane acrylate polymer, N-vinyl pyrrolidone and at least one additional material which may be a monofunctional acrylic monomer, a polyfunctional acrylic monomer or a polyfunctional polyester acrylate oligomer.
In advancing the art of producing transparent bullet proof laminates, it is evident that not only would skilled workers in this art seek to develop easier methods of producing the laminates, but that the demands on the effectiveness of the sandwiches would be further increased. Thus, those of the previously mentioned patents that provide instances of firing at test panels describe single shot results. GB 2098650 provides protection against multiple shots by combination of multiple layers. The transition layer is a thin, i.e. 0.86 mm thick, polysiloxane-polycarbonate copolymer which can provide little energy absorbing effect. Whilst little practical detail is given of the application of this layer into the sandwich, it can be assumed that it will suffer from the problems of air bubbles and two component curing problems previously discussed.
The problem of multiple firings is specifically a practical one that bullet resistant materials are likely to have to meet in cases where shots would be fired in close groupings. Thus, a first shot may severely damage the armor without exiting the opposite side, but could provide sufficient damage to enable a second bullet to exit. Therefore, greater resistance to penetration as well as smaller radii of damage are being sought for improved performance. GB 2098650 demonstrates an awareness and an attempt to achieve higher performance. Still, insofar as is known, the deficiencies of the prior art, such as noted above, have not been eliminated.